Toward better understanding of B/Ca and δ11B proxies: An experimental approach

Abstract:

The abundance and isotopic composition of boron (B/Ca and δ¹¹B) in marine biogenic carbonates is an important paleoceanographic tool to probe carbon cycling in the ocean-atmosphere system. These B-based proxies rely on a fundamental assumption that boron incorporation into carbonates occur via B(OH)₄^- adsorption with little isotopic fractionation, which is based on key results from the classic inorganic experiments performed in the late 1990s (e.g., Hemming et al., 1995, GCA, v59, 371-379; Sanyal et al., 2000, GCA, v64, 1551-1555).

However, a collection of new experimental data published in recent years consistently suggests a more complicated picture for fluid-crystal element and isotope partitioning of B into inorganic carbonates. For instance, we performed novel inorganic calcite precipitation experiments by systematically adjusting solution pH as well as total B, total DIC and Ca concentrations (Uchikawa et al., 2015, GCA, v150, 171-191), and the results showcased apparent kinetic effects related to precipitation rate on B/Ca. Moreover, the results also indicated a dependence of B/Ca on the concentration ratio of total B to total DIC, which was interpreted as indirect evidence for potential B(OH)₃ incorporation into synthetic calcite. Notably, relatively simple solutions of NaCl-CaCl₂-B(OH)₃ system were used for our previous experiments. This presentation features our latest results from similar experiments but using artificial seawater in order to close the gap between simplified experimental conditions to in-situ marine settings. Our preliminary results reveal a precipitation rate control even when artificial seawater is used for the experiments, making a strong case that kinetic effects on B/Ca are universal in inorganic carbonates. With the aid of new isotopic results, we also attempt to discuss possible scenarios of B incorporation pathway in inorganic systems.